ISO 3651 Determination of Resistance of Stainless Steels to Intergranular Corrosion

The “ISO 3651 Determination of resistance of stainless steels to intergranular corrosion” standard published by the International Organization for Standardization (ISO) consists of a number of subsections published under the same title. Namely,

• The standard "ISO 3651-1 Part I: Corrosion test in nitric acid medium by measurement of mass loss (Huey test)" describes a method for determining the resistance of austenitic stainless steels to intergranular corrosion in nitric acid medium by measurement of mass loss (Huey test). This standard also specifies the objectives that can be assigned to the test.

This method is only applicable to austenitic and ferritic-austenitic (duplex) stainless steels supplied as rolled or forged products, pipes and cast products and intended for use in a strongly oxidizing environment (e.g. relatively concentrated nitric acid). In general, the Huey test should not be used for grades containing molybdenum unless the material being tested will be used in nitric acid service.

The result of a corrosion test is only strictly valid for the corrosive medium used in the test. It provides a basis for estimating resistance to intergranular corrosion but cannot be used to check resistance to other forms of corrosion (such as general corrosion, pitting, stress corrosion). The user must adapt the specified corrosion test to the use to which the alloy is to be put. In no case should this test be considered an absolute measure of the quality of the alloy.

This intergranular corrosion test may have one of the following purposes. If an order specifies this corrosion test, the purpose of the test must be stated at the time of ordering.

• Verification of the alloy's intrinsic resistance to intergranular corrosion: This verification is only valid for austenitic steel grades specifically produced for resistance to intergranular corrosion in strongly oxidizing environments. The sample is examined after a heat treatment for sensitization.
• Investigation of the efficiency of the solution treatment: This examination is carried out only on thin products for which the cooling rate can be made sufficiently fast. The sample is examined as delivered to the user, without heat treatment for sensitization.

In principle, when performing the test, a prepared test piece is weighed, then immersed in a boiling nitric acid solution for 48 periods of 5 hours. The criterion for evaluating the test result is the mass loss determined by weighing after each period.

• The standard "ISO 3651-2 Part 2: Ferritic, austenitic and ferritic-austenitic (duplex) stainless steels - Corrosion testing in sulfuric acid environments" describes methods for determining the resistance of ferritic, austenitic and ferritic-austenitic (duplex) stainless steels to intergranular corrosion in sulfuric acid environments. This standard also specifies the objectives that can be assigned to the test.

The test methods included in this standard are:

• Method A: 16 percent sulfuric acid/copper sulfate test (Monypenny Strauss test)
• Method B: 35 percent sulfuric acid/copper sulfate test
• Method C: 40 percent sulfuric acid/ferric sulfate test.

These methods apply to stainless steels supplied as cast, rolled or forged products and tubes and intended for use in a mildly oxidizing acid environment (e.g. sulphuric acid, phosphoric acid). Unless specified in the product standard, the method to be used, A, B or C, is the subject of agreement between the parties concerned. The annex to this standard contains examples of the application of the three methods on stainless steels (Annex A).

The result of a corrosion test is strictly valid only for the corrosive medium used in the test. It provides a basis for estimating resistance to intergranular corrosion but cannot be used to check resistance to other forms of corrosion (such as general corrosion, pitting, stress corrosion). The user must adapt the specified corrosion test to the use to which the alloy is to be put. In no case should these tests be considered as an absolute measure of the quality of the metal.

This intergranular corrosion test may have any of the following purposes:

• Verification of the intrinsic resistance of steel to intergranular corrosion: This verification applies only to low-carbon steels (not more than 0,03 percent carbon) and stabilized steels specified for resistance to intergranular corrosion. The metal is examined after a heat treatment, which may be a sensitizing heat treatment or sensitizing welding.
• Investigation of the efficiency of the solution treatment: This investigation is carried out only on thin products for which the cooling rate can be made sufficiently fast. It is of interest only for steels not described above. The metal is examined as delivered to the user, without heat treatment for sensitization.

This method is applicable to stainless steels supplied as rolled sheets and welded tubes and intended for use in mildly oxidizing acid media. The result of the corrosion test is strictly valid only for the corrosive media used in the test. It provides a basis for estimating resistance to intergranular corrosion but cannot be used to check resistance to other forms of corrosion (such as general corrosion, pitting, stress corrosion). It is important that the user adapts the specified corrosion test where the steels are used. In no case is this test considered an absolute measure of the quality of the metal.

When performing the test, in principle, the test pieces should be prepared as specified in this standard, depending on the purpose of the test and the agreement between the supplier and the purchaser. The thickness of the test specimen may be the same as the original specimens. A specimen with a total surface area of ​​5 cm2 to 20 cm2 is recommended. It should include the welded zone, the heat affected zone and the unaffected base metal.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services in accordance with the "ISO 3651 Determination of the resistance of stainless steels to intergranular corrosion" standard are also provided to businesses.